The present invention relates both to a method for improving the primeness adhesion of cured silicone elastomers and to curable (and cured) high strength, organopolysiloxane molding compositions useful to produce cured silicone rubber articles that exhibit excellent adhesion to thermoplastic resins such as, but not limited to, PBT and PPA, while not significantly adhering to metal substrates.
Moldable primerless organopolysiloxane compositions are known and used. One problem with all such compositions is that due to the adhesion additives in the compositions they also adhere to metal surfaces. In molding processes, the typical mold is metal. It is difficult to selectively adhere to thermoplastic substrates without deleterious effects. Additionally, the kinetics and thermochemistry of the molding process and the compositions used in such processes are such that only small lightweight composite plastic and silicone elastomer articles of manufacture can be made by molding techniques because of the difficulty of removing the article from the metal mold.
Organopolysiloxane compositions useful for molding typically contain alkenyl polymers, fillers, and other additives. The compositions contain a platinum catalyst, a hydride crosslinker and cure inhibitors. These can be one or two component systems that are mixed prior to the molding process. These heat cured (i.e. high consistency or millable rubber) or liquid injection molded silicone rubbers are commonly available without additives that improve adhesion.
Typical molding compositions generally include the following ingredients:
1) 100 parts by weight of a vinyl stopped polydimethylsiloxane having a viscosity ranging from 10,000 to 1,000,000 centipoises at 25xc2x0 C. and a vinyl content ranging from about 0.01 to 0.25 mole per cent;
2) up to 10 parts by weight of a low molecular weight vinyl stopped vinyl on chain hydroxyl end blocked polyorganosiloxane having a viscosity ranging from 50 to 10,000 centipoises at 25xc2x0 C. and a vinyl content of 0.3 to 30 mole per cent;
3) up to 10 parts by weight of a low molecular weight hydroxy end blocked polyorganosiloxane having a viscosity ranging from 500 to 2,000 centipoises at 25xc2x0 C.;
4) from 5 to 50 parts by weight of a fumed or pyrogenic silica having a surface area ranging from 150 to 450 square m/gm;
5) up to 40 parts by weight of an organohydrogenpolysiloxane crosslinker containing at least two silicon-bonded hydrogens per molecule and having a viscosity ranging from 80 to 1,000 centipoises at 25xc2x0 C.;
6) from 2 to 500 wppm of a platinum hydrosilylation catalyst;
7) from 0.01 to 3.0 parts by weight of an inhibitor compound; and
8) from 100 to 1,000 wppm of silyl hydrides.
Additional components may include extending fillers, coloring agents, and additives to impart increased performance with respect to certain physical properties such as oil resistance, heat aging, abrasion resistance and the like.
The functional properties of the resulting rubber elastomer depend not only on the levels of components but also on the properties of the filler, additives, and type of curing catalyst. Consequently, the elastomer property profile is highly dependent on the chemical nature of the various constituent components as well as the relative proportions of those components. For example, a heat additive increases the temperature range of the resulting rubber. Such improvements however, do not necessarily increase the adhesion stability and some times fail to adhere after heat soak.
In seeking to increase the selective adhesion of silicone compositions to thermoplastic and thermoset substrates, it is frequently desirable to increase the ability of the silicone to be released from a mating part or surface for which the molded composite may come in contact. These two properties collide in that adhesion of the silicone to a thermoplastic or thermoset substrate is desired but ready release from another substrate is necessary for the device to function. For example, the fabrication of connecters requires adhesion of the grommet and facial seal to a thermoplastic closure and also requires that the silicone surface have sufficient lubricity to allow mating sides to fasten and unfasten with ease.
A typical method of improving the lubricity of a silicone composition is simply to add a liquid component to the formulation that has limited solubility in the elastomer and thus the compound bleeds out over time. Ideally such a material itself also has a lubricating quality. These components are typically selected from the group of compounds consisting of phenyl containing siloxanes such as copolymers of polydimethylsiloxanes, diphenylsiloxane or methylphenylsiloxane, and fluorosiloxanes such as silicones containing trifluoropropyl substituted siloxanes.
Accomplishment of this selective adhesion is by the addition to a diorganopolysiloxane composition of soluble polydiorganosiloxanes, polycycloorganosiloxanes (linear and cyclic) and/or hydroxy end blocked hydrocarbons (glycols) of a specific molecular distribution preferably having a viscosity of 10 to 1,000 centipoises at 25xc2x0 C.; and an epoxy functional compound, thus giving release characteristics to metal substrates while maintaining adhesion to the thermoplastic substrate.
In general, the preferred embodiment of the present invention provides for a curable silicone elastomer composition comprising:
1) a silicone elastomer;
2) an epoxy functional compound; and
3) a linear and/or cyclic diorganopolysiloxane and/or hydroxy end blocked hydrocarbons (glycols). Examples of such glycols include polyethylene glycol (e.g., PEG 400) and 1,3 butylene glycol PEG 400 is preferred.
The foregoing composition may also include a silicone soluble in the curable elastomer composition. Preferably, the silicone is less soluble in the curable elastomer composition when the curable composition has been cured. As a result, the silicone bleeds out of cured composition to improve lubricity.
The present invention provides a composition that selectively adheres to a substrate that may be a thermoset or a thermoplastic, but not to a metal such as aluminum or steel. The thermoset or thermoplastic substrates may be filled polymers with glass or clays. The silicone compositions of the present invention release readily from metal substrates after they have been overmolded, transfer molded or injection molded. The present invention further provides for the incorporation of an adhesion promoter and a self-bleed additive that allows lubrication of the surfaces of the cured silicone simultaneously with adhesion to the overmolded, transfer molded or injection molded substrate. The formulations of the present invention demonstrate that compositions containing both an adhesion promoter and a self-bleed additive for lubricity can simultaneously achieve the contradictory goals of adhesion to a substrate used in overmolding, transfer or injection molding and release from a second substrate where the silicone was not bonded to the substrate by the action of the adhesion promoter and the co-molding process of overmolding, transfer molding or injection molding.
This invention utilizes the release characteristics of soluble polydiorganosiloxanes, polycycloorganosiloxanes (linear and cyclic), and/or hydroxy end blocked hydrocarbons (glycols) of specific molecular distribution preferably having viscosity of 10 to 1,000 centipoises at 25xc2x0 C. The two (2) additives that provide the selective adhesion properties are:
1) generally from 0.01 to 30, preferably from 0.01 to 10, more preferably from 0.05 to 1.0 and even more preferably from 0.10 to 0.50, parts by weight of an epoxy function compound; and
2) generally from 0.01 to 10, preferably from 0.5 to 5 and more preferably from 1 to 3, parts by weight of soluble polydiorganosiloxanes and/or polycycloorganosiloxanes (linear and/or cyclic) and/or hydroxy end blocked hydrocarbons (glycols), or mixtures thereof.
Generally, it has been found that polybutylene glycols provide better release to metals during molding than poly-dimethyl siloxanes alone.
Optionally, a silicone soluble in the curable elastomer composition can be added. The silicone is selected such that it is less soluble in the curable elastomer composition when the curable composition has been cured. As a result, the silicone bleeds out of the cured composition.
In general, one preferred embodiment of the present invention comprises a curable organopolysiloxane composition having selective adhesion to a metal substrate. The composition should preferably include the following components:
(A) up to 100 parts by weight of an organopolysiloxane polymer having a viscosity of about 10,000 to about 10,000,000 centipoises at 25xc2x0 C. with sufficient vinyl, or mixtures of such organopolysiloxane, to provide functional reactivity to the following crosslinker;
(B) from 0.3 to 40 parts by weight of an organohydrogenpolysiloxane crosslinker containing at least two silicon-bonded hydrogens per molecule;
(C) a catalytically effective amount of a platinum group metal catalyst;
(D) from 0.01 to 3 parts by weight of a cure inhibitor, preferably of the type ethynyl cyclohexanol;
(E) from 0.01 to 30 parts by weight of an adhesion promoter compound having at least one hydroxy group and in the same molecule at least one substituent selected from the group consisting of silicon hydride, alkenyl, and acryl;
(F) from 0.01 to 10 parts by weight of an epoxy functional compound; and
(G) up to about 10 parts by weight based on the total composition of a compound selected from the group consisting of soluble polydiorganosiloxanes, polycycloorganosiloxanes (linear and cyclic) and/or hydroxy end blocked hydrocarbons (glycols) and having a specific molecular distribution such that the viscosity is 50 to 1,000 centipoises at 250xc2x0 C.
Preferably, the organopolysiloxane polymer contains at least two silicon-bonded lower alkenyl groups in each molecule and is a straight chain organopolysiloxane having a viscosity of about 100,000 to about 10,000,000 centipoises at 25xc2x0 C., or mixtures of such organopolysiloxanes, and the lower alkenyl groups are vinyl. Examples of organopolysiloxanes include Dow Corning Q42901 and Q42903 gums, with an average pendant vinyl content ranging from 0.01 to 0.20 mole percent with 0.80 to 0.20 mole percent being preferable.
Preferably, the organohydrogenpolysiloxane crosslinker has organic substituents bonded to silicon atoms, which are more preferably methyl, and the concentration of organohydrogenpolysiloxane provides 0.002 to 0.0002 moles of silicon-bonded hydrogen atoms for each mole of silicon-bonded alkenyl radicals in the organopolysiloxane polymer, and wherein the crosslinker has the formula:
R13Sixe2x80x94(OSiH(R2))mxe2x80x94(OSiR22)nxe2x80x94OSiR13
wherein R1 is independently chosen from a hydrogen or monovalent hydrocarbon radical free of aliphatic unsaturation containing 1 to about 8 carbon atoms, R2 is independently chosen from a monovalent hydrocarbon radical free of aliphatic unsaturation containing 1 to about 4 carbon atoms, m is one or more, n is one or more, and m+n varies so that the crosslinker has a viscosity ranging from about 80 to 1000 centipoise at 250xc2x0 C. In addition, R1 can itself be a polydiorganosiloxane group which is a straight chain. Where R1 represents a polydiorganosiloxane, it may include vinyl groups and the crosslinker has a viscosity of 10,000 to 10,000,000 centipoise at 25xc2x0 C. The crosslinker may also comprise mixtures of crosslinkers that fall within the foregoing description. Examples of crosslinkers include Dow Coming 1107 and 63570 cross-linkers with Dow Corning SYL-OFF(copyright) 7678 being preferable.
The platinum group metal catalyst is selected from the group consisting of platinum metal, platinum compounds, platinum complexes and mixtures thereof. Preferably, the platinum group metal catalyst is chloroplatinic acid complexed with a liquid olefin or an organosiloxane containing ethylenically unsaturated hydrocarbon radicals bonded to silicon. More preferably, the platinum group metal catalyst is a neutralized complex of chloroplatinic acid or platinum dichloride with divinyltetramethyldisiloxane.
In general, the platinum group metal catalyst provides about 0.1 to about 500 parts by weight platinum group metal per million parts of the combined weights of organopolysiloxane and organohydrogenpolysiloxane. Preferably, the platinum group metal catalyst provides about 1 to about 50 parts by weight platinum group metal per million parts of the combined weights of organopolysiloxane and organohydrogenpolysiloxane.
In general, the cure inhibitor compound is an acetylene alcohol derivative such as ethynyl cyclohexanol or pyridine alcohol having the following general formula:
Rxe2x80x2R(OH)x
Where Rxe2x80x2 is an alkyne unsaturated hydrocarbon chain, branched or unbranched, and R is a saturated hydrocarbon linear or cyclic with hydroxy groups pendant or end blocked with hydroxy saturation indicated by x, which is 1 to complete hydroxy saturation of the R group.
In general, the adhesion promoter is a hydroxinated silicone compound of the formula:
(OH)(R1)2SiO(xe2x80x94SiO(R2)2xe2x80x94)zSi(R1)2(OH)
Where R1 is any hydrocarbon alkyl or alkenyl radical linear or branched comprising not more than 10 carbon atoms, R2 is any mixture of any alkyl, alkenyl, aliphatic, or aromatic radical, linear or branched, up to 12 carbon atoms, and where z is of sufficient number to create a viscosity of about 50 to about 10,000 centipoises at 25xc2x0 C. In general from 0.01 to 30, preferably from 0.05 to 2, and more preferably from 0.5 to 1, parts per weight are used. Examples include 1-hydroxy 2,3 methyl endblocked vinylmethyl siloxane (DP 10 to 12 with 2 to 4 pendant vinyl groups).
In general, the epoxy-functional compound is glycidoxypropyltrialkoxysilane having the formula: 
wherein R denotes a methyl or ethyl radical. The embodiment wherein R is a methyl group is particularly preferred. Glycidoxypropyltrimethoxysilane may also include quantities of methacrylvinylpolysilane, chlorotrimethoxysilane, vinyltrimethoxysilane, and divlnyldimethoxysilane. In general, from 0.01 to 30, preferably, from 0.05 to 10, and more preferably, from 0.1 to 0.05 parts by weight are used.
The polydiorganosiloxanes include Dow Corning 200 fluid. Viscosity grades 50 to 200 are preferable and grades 50 to 10,000 in general. An example of polycycloorganosiloxanes includes Dow Corning 244 fluid and other cyclic siloxanes from D4 to D10 (DP units), preferably D 4 and D5. The molecular distribution should be such as to produce a viscosity at 25xc2x0 C. of 50 to 1,000, preferably 50 to 500 centipoises, and more preferably 50 to 200 centipoises.
In another embodiment of the present invention, the epoxy-functional compound is glycidoxypropyltrimethoxysilane. Preferably, the epoxy-functional compound comprises about 0.01 to 10 parts by weight per 100 weight parts of organopolysiloxane polymer.
The composition according to the present invention can include a filler selected from the group consisting of fumed silica, treated fumed silica, precipitated silica, treated precipitated silica, fused silica, and finely divided quartz and mixtures thereof.
In yet another embodiment of the present invention, a composite is formed having bonded thereto a cured organopolysiloxane composition. The composition preferably contains the following components:
(A) up to 100 parts by weight of an organopolysiloxane polymer having a viscosity of about 10,000 to about 10,000,000 centipoises at 25xc2x0 C. with sufficient vinyl, or mixtures of such organopolysiloxane to provide functional reactivity of a crosslinker organohydrogenpolysiloxane;
(B) from 0.3 to 40 parts by weight of an organohydrogenpolysiloxane crosslinker containing at least two silicon-bonded hydrogens per molecule;
(C) a catalytically effective amount of a platinum group metal catalyst;
(D) from 0.01 to 3 parts by weight of a cure inhibitor, preferably of the type 4 or 5, ethyl cyclohexan-1-ol;
(E) from 0.01 to 30 parts by weight of a compound comprising at least one hydroxy group and in the same molecule at least one substituent selected from the group consisting of silicon hydride, alkenyl, and acryl;
(F) from 0.01 to 10 parts of an epoxy functional compound; and
(G) up to about 10 parts by weight based on the total composition of a compound selected from the group consisting of a soluble polydiorganosiloxanes, polycycloorganosiloxanes (linear and cyclic) and/or hydroxy end blocked hydrocarbons (glycols) and having a specific molecular distribution such that the viscosity is 50 to 1,000 centipoises at 25xc2x0 C.
In still yet another embodiment of the present invention, a composite containing an epoxy-coated substrate having bonded thereto a cured organopolysiloxane composition is formed. The composition preferably comprises:
(A) up to 100 parts by weight of an organopolysiloxane polymer having a viscosity of about 10,000 to about 10,000,000 centipoises at 25xc2x0 C. with sufficient vinyl, or mixtures of such organopolysiloxane to provide functional reactivity of the following crosslinker;
(B) from 0.3 to 40 parts by weight of an organohydrogenpolysiloxane crosslinker containing at least two silicon-bonded hydrogens per molecule;
(C) a catalytically effective amount of a platinum group metal catalyst;
(D) from 0.01 to 3 parts by weight of a cure inhibitor, preferably of the type 4 or 5, ethyl cyclohexan-1-ol;
(E) from 0.01 to 30 parts by weight of a compound comprising at least one hydroxy group and in the same molecule at least one substituent selected from a group consisting of silicon hydride, alkenyl and acryl;
(F) from 0.01 to 10 parts of an epoxy functional compound; and
(G) up to about 10 parts by weight based on the total composition of a compound selected from the group consisting of soluble polydiorganosiloxanes, polycycloorganosiloxanes (linear and cyclic) and/or hydroxy end blocked hydrocarbons (glycols) and having a specific molecular distribution such that the viscosity is 50 to 1,000 centipoises at 250xc2x0 C.
In a preferred embodiment, the substrate is metal. The epoxy coating comprises chemistries used in cast, form in place, or vacuum bag cures. Moldable epoxy composites are preferable.
Preferably, a silicone soluble in the curable elastomer composition is added. In general, the silicone is less soluble in the curable elastomer composition when the curable composition has been cured. In such case, the silicone bleeds out of the cured composition. Preferably, the silicone is selected from the following or mixtures thereof:
(A) phenyl containing siloxanes such as copolymers of diphenylsiloxane with diorganosiloxanes and copolymers of methylphenylsiloxane with diorganosiloxanes, more preferably dimethylsiloxanes, having a viscosity ranging between about 10 to about 10,000 centipoises at 25xc2x0 C., preferably ranging from about 100 to about 1000 centipoises at 250xc2x0 C.; and
(B) fluorosiloxanes such as silicones containing trifluoropropyl substituted siloxanes, having a viscosity ranging between 100 to about 10,000 centipoises at 25xc2x0 C., preferably ranging from about 100 to about 1000 centipoises at 25xc2x0 C.
Examples of the foregoing include phenyl siloxanes commonly sold as Dow Corning 550 Fluid or GE SF-1154 and those for fluorosiloxanes FS-149, FS150-10M FS157 all of various viscosities and purity grades. These components lubricate without adversely affecting the adhesion of the composition to a substrate.
The resins to which the cured inventive composition can be bonded include Nylon, polyphenylene, polyphthalamide, polyphenylene ether, polyamide and polyimid thermoplastic resins that may or may not be glass filled, mineral filled, or carbon filled. Further, thermoset resins such as epoxy, silicone or others that are manufactured by coating a substrate or by direct casting can be bonded to the cured organopolysiloxane composition in of the present invention. Preferably the resin is PBA or PBT.
Additional components may include extending fillers, coloring agents, and additives to impart increased performance with respect to certain physical properties such as oil resistance, heat aging, abrasion resistance and the like.
All such compositions have selective adhesion to polymer substrates and do not have significant adhesion to metal surfaces. The significance of selective adhesion preferably is such that the adhesion to metal in general is such that the composition sticks to a resin or epoxy coated substrate, but not to metal. Preferably, the adhesion to metal is less than half the adhesion to the target polymer substrate and more preferably has no cohesion failure to the metal as determined by lap shear testing.